The present invention relates to the field of submerged apparatus, in particular submerged cable systems as used for example in under sea communications involving the transfer of data through fibre optic cables located at the bottom of the sea. The cables referred to normally lie either on the sea bed, as is typically the case for deep water installations or they may be buried, beneath the sea bed, either by a natural process or by deliberate action, which may be a method typically employed in shallow water installations. For example, when a cable passes from deep water to a land base, as the shore is approached the cables may be buried beneath the sea bed and continue to be buried under ground when on land to protect them from mechanical damage, interference and such like. The invention not only applies to undersea applications, involving for example fibre optic carrying cables, but also applies to any underwater cable installation where an earth is required, including installations in lakes and such like.
The cables used, for example, in such underwater applications and which involve the carrying of optical fibres, generally require increased mechanical protection against damage in shallow water as compared to deep water, due to the higher likelihood of mechanical damage in shallower waters. Mechanical protection in the first instance typically involves the use of an insulated polyethylene sheath casing around a core where the various fibres power conductors are situated. However this may be supplemented by one or more layers of steel armouring wires, which is itself protected from the environment, for example from sea water, by an outer sheath or layer of tar or such like.
One of the major environmental hazards for this kind of cable is the presence of molecular hydrogen in the vicinity of the cable, which may for example be the result of local electrolysis of the sea water, such as that created by the close proximity of an earthing electrode, when operated as a cathode, through which electrical current is flowing to earth via the sea. This problem is exacerbated in situations where the molecular hydrogen is able to accumulate due to a lack of water movement, for example when a cable is buried beneath the sea or when a cable has become covered in silt or similar deposits.
One of the protective measures employed in submerged cables which carry in their core optical fibres and power conductors, is the use of a metallic layer, typically copper and which is located between the core of the cable and the polyethylene sheath. This metallic layer, which surrounds optical fibres in the core of the cable, serves the function, amongst other things, of providing a hydrogen barrier. Provided the metallic layer remains intact, it will prevent hydrogen from penetrating to the core of the cable, where it could damage optical fibres typically noticeable in the form of increasing optical losses.
In the field of submerged fibre optic cable communications, there is a requirement, owing to the long distances involved, to interrupt the cables at regular intervals in order to provide e.g. repeater units, equalisation units, as well as junctions where fibre optic paths are split, combined, rings completed and such like. These latter devices are generally referred to as branching units (BU's) and by the nature of the way in which they function, will often require an earth, for example to discharge into the water the DC powering current from one cable where three cables are joined together. For example, at a submerged branching unit where three cables meet, a line current of 1–2 amps from one of the three legs of the junction would typically need to flow to earth via the sea. This produces major local electrochemical effects in terms of hydrogen generation and/or metal corrosion from which the system needs to be protected.
One solution currently employed for providing an earth at a branching unit, is to insulate the system earth from the main housing of the branching unit and to connect it directly to a separate electrode, typically made of copper or steel which is placed in the water, either on or in the vicinity of the branching unit.
Such an arrangement has drawbacks, however, since, if the earth is operated as a cathode, it will be an intense source of molecular hydrogen. Therefore it cannot be placed less than about 2 metres from any non-hermetic part of the cable, including cable joint housings or copper weld skips. As regards the effects of such an intense source of molecular hydrogen on any fibre optic carrying cable in the vicinity, then any hydrogen path between such a source, which is in effect a region where hydrogen has a high partial pressure, and the core of the cable, where power and fibre optic cables are located, can result, albeit after several years of progressive penetration, in the failure of the fibre optic cables. This is typically noticeable in the form of progressively increasing optical losses.
However, it has been found that even the presence of a metallic layer in the cable, serving as a hydrogen barrier, cannot be totally relied on when the cable is in close proximity to an intense source of molecular hydrogen. The reason is that any defect in the copper layer, (which could typically arise as a result of bending the cable and other stresses arising from moving the cable, laying the cable, but also when cables are lifted to carry out repairs, such as may be required to gain access to a branching unit in order to repair a cable or repeater or other device which has been damaged or become defective) may allow hydrogen to penetrate. This drawback applies likewise when an earthing electrode, operated as a cathode, is located in close proximity to a branching unit housing, its associated bend limiting devices (otherwise referred to as an ‘armadillo’) and extremity termination boxes, since the seals of these may not effectively protect the components inside, including fibre optic cables within the housing, from the effects of penetration of molecular hydrogen with the passage of time. This means in practice that such an earth cannot be placed within less than about 2 metres from any non-hermetic part of the system.
If the electrode is in close proximity to any metal, then another drawback occurs, namely that both the electrode itself as well as any metalwork in the vicinity of the electrode, may be subject to very high levels of corrosion. Typically, any metallic parts within less than about 1 metre from an electrode operated as a cathode in an undersea application, may be subject to generally unacceptably high levels of corrosion due to earth currents flowing preferentially into them. In addition it is not possible to operate such an electrode as an anode because of metal erosion from the electrode itself.
A further drawback of this solution is that the electrode, which is installed on or near the cable housing, can give rise to handling problems in the cable factory and in marine situations, such as maneuvering the cable off a drum, cable laying and such.
To overcome some of the above mentioned problems, an alternative approach has sometimes been adopted. In this approach, the earth connection is made directly to the metallic BU housing and this housing is in turn electrically connected to the steel armouring wires on one or more of the three cable ‘legs’. With this method, all metallic parts are at the same potential so that no corrosion occurs due to earth-return currents. The BU housing is coated with an insulating material, whereas the armouring wires are in close contact with the sea, so that the BU earth current flows to sea through the armouring wires, with no build-up of hydrogen near any critical parts. This system works very well in shallow water, where the armouring wires are many kilometres long so that the hydrogen produced is distributed over a large area and so there is no build-up sufficient to have any significant effect on critical areas. In deep water, however, where the cable is not armoured, special lengths (˜50 m) of armouring need to be applied to the cable specifically for this purpose. In this situation, the armour wires need to be terminated at least 2 m from the BU and jointing boxes to avoid the hydrogen build-up problems mentioned earlier. Such an earth design should operate indefinitely when used as a cathode, and can be used as an anode for a time of the order of one year before corrosion of the armouring wires or other metallic parts becomes unacceptable.
If this latter design philosophy for BU earthing is adopted, hydrogen and corrosion problems are avoided, but there are a number of disadvantages, notably: the mechanical discontinuity in flexural stiffness of the cable near the BU gives major handling problems in factories and on board ships during installation or recovery, deep-water and shallow-water designs are different so that common spares are not possible, and the factory assembly process is a long and non-standard operation. To overcome these difficulties, a new approach is proposed.